In order for electronic devices to communicate, a wireless or wired protocol (i.e., standard) defines hardware and software parameters that enable the devices to send, receive, and interpret data. Frequency division multiplexing or frequency division modulation (FDM) is a technology that transmits multiple signals simultaneously over a single transmission path, such as a cable or wireless system. Each signal travels within its own unique frequency range (carrier), which is modulated by data (e.g., text, voice, video, etc.).
Orthogonal FDM (OFDM) distributes the data over a large number of carriers that are spaced apart at precise frequencies. To mitigate multi-path energy and to enable a transmitter and receiver to switch between different frequency bands some wired and wireless protocols define a guard interval that is placed before transmitted information symbols as a prefix. Other protocols define a guard interval that is placed after transmitted information symbols as a suffix (e.g., a zero-padded suffix or “ZPS”). In either case, the guard interval reduces interference between information symbols by providing time for multi-path reflections to attenuate.
FIG. 1a illustrates zero padded suffix operations at a transmitter and receiver. When a ZPS is used (e.g., in a Multi-Band Orthogonal Frequency Division Multiplexing (OFDM) system), a receiving device may use an “overlap-and-add” operation to add samples of the ZPS to samples of the symbol preceding the ZPS. For example, the overlap-and-add operation may combine the “N” samples of the ZPS with the first N samples of the symbol preceding the ZPS. The overlap-and-add operation may result in too much noise being added to the symbol.
There are a variety of techniques for representing noise at a receiver. FIG. 1b illustrates an I, Q plane for a quaternary phase shift key modulation (QPSK) scheme which has a constellation of four ideal possible symbols So, S1, S2, and S3 and received signal samples ‘x’ that are not aligned with the ideal possible symbols. The ‘x's’ next to each of the ideal symbols illustrate the common situation that occurs at a receiver that receives a QPSK signal that has passed through a noisy channel. The received modulated signal samples are not aligned with the ideal constellation points. For example, received symbol 104 is not aligned with ideal constellation symbol S3 102. The error distance 106 between the vector 108 to received symbol 104 and the vector 109 to ideal symbol S3 is representative of the noise that led to the misalignment. An error vector magnitude (EVM) metric is defined as the square root of the mean of the square of the error distances for received symbol samples around each of the ideal constellation symbols. The EVM is calculated per frame of received symbols or some other period of time. The EVM may be expressed in dB or as % RMS (root-mean-square) of the signal strength. EVM can be calculated for modulation schemes other than QPSK.
Typically, the greater the noise that is affecting modulated signals the greater the error distance between received signal samples and the ideal symbol with which they are associated. The greater the error distance the more difficult it becomes to map received signal samples to their associated ideal symbol and may prevent communication from occurring in some instances. Consequently, the problem of relatively too much noise being added during overlap-and-add operation is a significant and very substantial issue that needs addressing with solutions that overcome the deficiencies of the prior art.